Airworthiness Directives; Boeing Model 737-300, -400, and -500 Series Airplanes

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

[Docket No. 94-NM-158-AD]

Airworthiness Directives; Boeing Model 737-300, -400, and -500

Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This document proposes the adoption of a new airworthiness

directive (AD) that is applicable to certain Boeing Model 737-300, -

400, and -500 series airplanes. This proposal would require the

application of an improved topcoat material to the insulation of the

inner wall of the fan duct cowl (the firewall) of the thrust reversers.

This proposal is prompted by tests, which demonstrated that flames can

penetrate the firewall if certain combinations of insulation and

topcoat materials are used. The actions specified by the proposed AD

are intended to prevent failure of the fireproof insulation topcoat

installed on the firewalls of the thrust reverser fan cowls, which

could result in degradation or loss of the firewall and lead to an

uncontained engine fire.

DATES: Comments must be received by February 6, 1995.

ADDRESSES: Submit comments in triplicate to the Federal Aviation

Administration (FAA), Transport Airplane Directorate, ANM-103,

Attention: Rules Docket No. 94-NM-158-AD, 1601 Lind Avenue, SW.,

Renton, Washington 98055-4056. Comments may be inspected at this

location between 9 a.m. and 3 p.m., Monday through Friday, except

Federal holidays.

The service information referenced in the proposed rule may be

obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle,

Washington 98124-2207. This information may be examined at the FAA,

Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton,

Washington.

FOR FURTHER INFORMATION CONTACT: Stephen Bray, Aerospace Engineer,

Propulsion Branch, ANM-140S, Seattle Aircraft Certification Office,

FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton,

Washington 98055-4056; telephone (206) 227-2681; fax (206) 227-1181.

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to participate in the making of the

proposed rule by submitting such written data, views, or arguments as

they may desire. Communications shall identify the Rules Docket number

and be submitted in triplicate to the address specified above. All

communications received on or before the closing date for comments,

specified above, will be considered before taking action on the

proposed rule. The proposals contained in this notice may be changed in

light of the comments received.

Comments are specifically invited on the overall regulatory,

economic, environmental, and energy aspects of the proposed rule. All

comments submitted will be available, both before and after the closing

date for comments, in the Rules Docket for examination by interested

persons. A report summarizing each FAA-public contact concerned with

the substance of this proposal will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments

submitted in response to this notice must submit a self-addressed,

stamped postcard on which the following statement is made: ``Comments

to Docket Number 94-NM-158-AD.'' The postcard will be date stamped and

returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this NPRM by submitting a request

to the FAA, Transport Airplane Directorate, ANM-103, Attention: Rules

Docket No. 94-NM-158-AD, 1601 Lind Avenue, SW., Renton, Washington

98055-4056.

Discussion

The topcoat material used on the firewall insulation of the thrust

reverser fan duct cowls installed on Boeing Model 737 series airplanes

equipped with CFM 56-3 series engines was changed in 1991 to decrease

weight. (This change was considered to be a product improvement.) At

that time, Dow Corning DC92-010 topcoat material was approved as an

optional fireproof topcoat, that could be used in lieu of the

previously used RTV60 topcoat material, for use on MA-25S and Crown-

Metro 64-C1-2 insulation located on the inner surface of the firewall

panel. The qualification testing of Dow Corning DC92-010 topcoat

material included fire tests of panels on which this material had been

applied on insulation other than MA-25S and Crown-Metro 64-C1-2

insulation materials.

In November 1991, Boeing began using Dow Corning DC92-010 topcoat

material on the insulation of the fan duct inner wall and upper

bifurcation of the thrust reverser fan duct cowls installed on Model

737-300, -400, and -500 series airplanes equipped with CFM 56-3 series

engines. Subsequent burner rig development testing, however, indicated

that this change to Dow Corning DC92-010 topcoat material had adversely

affected the performance of the firewall panels that were insulated

with MA-25S and Crown Metro 64-C1-2 insulation materials. The test

panels that were insulated with MA-25S material and those insulated

with Crown Metro 64-C1-2 successfully withstood a 2000 degrees

Fahrenheit flame for 15 minutes with backside airflow, as is required

by part 25 of the Federal Aviation Regulations (FAR) (14 CFR part 25).

However, during static testing of the panels, which is also required by

the part 25 of the FAR, flame penetrated the panels insulated with MA-

25S at approximately 5 minutes into the test, due to mud cracking and

fissuring of the insulation. The panels insulated with Crown Metro 64-

C1-2 material passed this static fire test.

Failure of the fireproof insulation topcoat installed on the

firewalls for the thrust reverser fan cowls can result in degradation

or loss of the firewall, which could lead to an uncontained engine

fire.

In light of the results of this testing, and in order to ensure the

integrity of the engine cowl firewall on the affected Model 737 series

airplanes, Boeing discontinued using Dow Corning DC92-010 topcoat

material on airplanes in production as of February 1992. The RTV60

topcoat material has been used on airplanes produced as of that date.

Additionally, Boeing developed a repair configuration for those

Model 737's that were delivered with the Dow Corning DC92-010 topcoat

material. This repair consists of the application of a improved topcoat

material on the existing surface.

The FAA has reviewed and approved Boeing Alert Service Bulletin

737-78A1056, dated August 11, 1994, which describes procedures for

applying an improved fireproof topcoat on the fire wall of the thrust

reverser fan duct cowls. The topcoat is comprised of a layer of Nextel

fabric embedded in another layer of Dow Corning DC92-010, which is

applied over the existing surface.

Since an unsafe condition has been identified that is likely to

exist or develop on other products of this same type design, the

proposed AD would require application of an improved fireproof topcoat

on the fire wall of the thrust reverser fan duct cowls. The actions

would be required to be accomplished in accordance with the service

bulletin described previously.

This proposed rule would be applicable only to airplanes having

line numbers 2137 through 2271, inclusive. Beginning with line number

2272, Dow Corning DC92-010 topcoat material was no longer used as a

topcoat on the insulation of the subject firewalls.

There are approximately 135 Model 737-300, -400, and -500 series

airplanes of the affected design in the worldwide fleet. The FAA

estimates that 18 airplanes of U.S. registry would be affected by this

proposed AD, that it would take approximately 13 work hours per

airplane to accomplish the proposed actions, and that the average labor

rate is $60 per work hour. Required parts would be provided by the

manufacturer at no charge to the operators. Based on these figures, the

total cost impact of the proposed AD on U.S. operators is estimated to

be $14,040, or $780 per airplane.

The total cost impact figure discussed above is based on

assumptions that no operator has yet accomplished any of the proposed

requirements of this AD action, and that no operator would accomplish

those actions in the future if this AD were not adopted.

The regulations proposed herein would not have substantial direct

effects on the States, on the relationship between the national

government and the States, or on the distribution of power and

responsibilities among the various levels of government. Therefore, in

accordance with Executive Order 12612, it is determined that this

proposal would not have sufficient federalism implications to warrant

the preparation of a Federalism Assessment.

For the reasons discussed above, I certify that this proposed

regulation (1) is not a ``significant regulatory action'' under

Executive Order 12866; (2) is not a ``significant rule'' under the DOT

Regulatory Policies and Procedures (44 FR 11034, February 26, 1979);

and (3) if promulgated, will not have a significant economic impact,

positive or negative, on a substantial number of small entities under

the criteria of the Regulatory Flexibility Act. A copy of the draft

regulatory evaluation prepared for this action is contained in the

Rules Docket. A copy of it may be obtained by contacting the Rules

Docket at the location provided under the caption ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Safety.

The Proposed Amendment

Accordingly, pursuant to the authority delegated to me by the

Administrator, the Federal Aviation Administration proposes to amend

part 39 of the Federal Aviation Regulations (14 CFR part 39) as

follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C.

106(g); and 14 CFR 11.89.

Sec. 39.13 [Amended]

2. Section 39.13 is amended by adding the following new

airworthiness directive:

Boeing: Docket 94-NM-158-AD.

Applicability: Model 737-300, -400, and -500 series airplanes;

line numbers 2137 through 2271, inclusive; certificated in any

category.

Note 1: This AD applies to each airplane identified in the

preceding applicability provision, regardless of whether it has been

modified, altered, or repaired in the area subject to the

requirements of this AD. For airplanes that have been modified,

altered, or repaired so that the performance of the requirements of

this AD is affected, the owner/operator must use the authority

provided in paragraph (b) to request approval from the FAA. This

approval may address either no action, if the current configuration

eliminates the unsafe condition; or different actions necessary to

address the unsafe condition described in this AD. Such a request

should include an assessment of the effect of the changed

configuration on the unsafe condition addressed by this AD. In no

case does the presence of any modification, alteration, or repair

remove any airplane from the applicability of this AD.

Compliance: Required as indicated, unless accomplished

previously.

To prevent the failure of the fireproof insulation topcoat

installed on the firewalls for the thrust reverser fan cowls, which

can result in degradation or loss of the firewall and lead to an

uncontained engine fire, accomplish the following:

(a) Within 24 months after the effective date of this AD,

inspect the inner wall of the fan duct cowl (the firewall) of the

thrust reversers to determine the type of topcoat material

installed, in accordance with Boeing Alert Service Bulletin 737-

78A1056, dated August 11, 1994.

(1) If the existing topcoat has silica fibers in it, no further

action is required by this AD.

(2) If the existing topcoat does not have silica fibers in it,

prior to further flight, accomplish the application of a the DC92-

010 topcoat to the firewall of the thrust reversers in accordance

with the service bulletin.

(b) An alternative method of compliance or adjustment of the

compliance time that provides an acceptable level of safety may be

used if approved by the Manager, Seattle Aircraft Certification

Office (ACO), FAA, Transport Airplane Directorate. Operators shall

submit their requests through an appropriate FAA Principal

Maintenance Inspector, who may add comments and then send it to the

Manager, Seattle ACO.

Note 2: Information concerning the existence of approved

alternative methods of compliance with this AD, if any, may be

obtained from the Seattle ACO.

(c) Special flight permits may be issued in accordance with

sections 21.197 and 21.199 of the Federal Aviation Regulations (14

CFR 21.197 and 21.199) to operate the airplane to a location where

the requirements of this AD can be accomplished.

Issued in Renton, Washington, on December 2, 1994.

James V. Devany,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-30142 Filed 12-7-94; 8:45 am]

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